New Combi boiler and hot water (lack of)

[DavidG]

Newbie post from a long time lurker here. Hoping someone can help.

As part of a loft conversion, we replaced the old boiler system.

The old was a 'traditional' open vented boiler system with a pump and a hot water tank, a salamander shower pump and a Bensham thermostatically controlled shower mixer. The new is a Worcester Bosch (WB) 28CDi RSF combi in the loft.

Since installation (by a plumber working for the building contractor) the heating has been fine, but the 'hot' water has been tepid. The plumber went through the trouble-shooting routine, said that a flow sensor was defective and called WB. Today the WB guy came, tested everything. He says the sensor is fine but that we had a 'cross'leak'. He thinks that the shower mixer is allowing cold water up the hot side and into the taps (which he tested by switching off the hot supply and getting a good flow of cold water out of the bath hot tap).

The local bathroom place says this is a good quality mixer suitable for mains pressure and disputes the diagnosis. Any thoughts please - just trying to make sure i'm not getting dicked around and looking forward to a hot shower again.

[Doctor Evil]

Is the shower mixer fitted properly, the hot pipe to the hot connection?

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[DavidG]

I think so yes, because we are getting tepid (not cold) water out of it. The cold water from the cold tap in the sink is much colder.

[Ed Sirett]

The diagnosis by WB guy is plausible. Quite a few but not all shower mixers and bath-shower mixers have this 'feature'. A number of manufacturers but not all put non-return valves in the inlet to the mixer to ove come this 'feature'.

The shop may be right in saying what they say about the mixer but that won't stop the problem in this case.

As a temporary work around try turning the mixer all the way to full hot or full cold and see if the 'feature' goes away. If you have the sort that only have one control that goes from off to cold through hot then you can't, off course. One or maybe two non-return valves added to the supply to the mixer will fix it properly.